X-ray photographing is used for medical diagnosis, non-destructive inspection for industrial purpose and the like. An intensifying screen/film method is used generally for the X-ray photographing. The intensifying screen/film method is a method which converts an X-ray passed through a subject into visible light on an intensifying screen and photosensitizes a film by the visible light to obtain an X-ray image. The medical diagnosis and non-destructive inspection are carried out based on the X-ray image formed on the film.
Meanwhile, with the recent advance of digital technology, a method which converts an X-ray image into an electric signal and processes the electric signal into an image so as to examine is now becoming popular. The medical diagnosis and non-destructive inspection are carried out on the basis of a visible image (X-ray image) displayed on a CRT or the like. As a method of examining an X-ray image by digitizing it, there is known a method which uses an X-ray image intensifier (I.I. tube) as an X-ray detector.
The X-ray image intensifier is provided with an input section which converts an X-ray into photoelectrons and an output section which converts the photoelectrons into visible light at both ends of a vacuum envelope. The output visible light image is digitized by a CCD camera or the like. For example, lungs are diagnosed by photographing a region of about 400×400 mm, and to detect an examination image of such a photographed region by the CCD camera sized about one inch, it is necessary to highly collect light within the I.I. tube. Therefore, there are problems that the I.I. tube becomes large in size and the like.
With the advance of the semiconductor processing technology in these years, there is developed a system that X-ray photographing is carried out using semiconductor sensors and semiconductor elements arranged in the form of an array. This type of X-ray imaging system can easily deal with a large photographing region and can directly digitize image data. Therefore, it is suitable for creation of database of X-ray photographed data (image data) in, for example, the medical field. In addition, it has advantages that a dynamic range is large in comparison with that of X-ray photographing systems using a conventional photosensitive film, and image accuracy is hardly affected by a change in X-ray exposure amounts.
In the X-ray imaging system using the semiconductor process technology, a planar X-ray detector with an amorphous silicon thin film transistor (a-Si TFT) in the form of an array applied to a switching gate is used. As the planar X-ray detector, there are known a direct conversion method using an X-ray-electric charge conversion film to convert an X-ray image directly into electric charge information and an indirect conversion method using a phosphor layer for converting an X-ray image into a light signal and a photoelectric conversion film for converting the light signal into electric charge information.
Sensitivity obtained by the planar X-ray detector of the indirect conversion method is equivalent to or less than that of a photographing system using, for example, the intensifying screen/film method or an X-ray image intensifier. Therefore, the planar X-ray detector of the direct conversion method which can provide high sensitivity is watched with interest in these years. For the planar X-ray detector of the direct conversion method, it is considered to use a material such as a-Se, PbI2, HgI2 or the like for the X-ray-electric charge conversion film (see Japanese Patent Laid-Open Applications Nos. 2001-264443 and 2001-320035).
PbI2 and HgI2 in the constituent materials of the above-described X-ray-electric charge conversion film are worried that Pb and Hg exert an adverse effect on human bodies and environments and, therefore, it is demanded to decrease the use of them in various fields. Even Se is worried that it exerts an adverse effect on human bodies and environments if it is contained in a large amount. In addition, it is necessary to form the X-ray-electric charge conversion film uniformly in order to obtain an accurate image by the planar X-ray detector. But, the conventional X-ray-electric charge conversion film materials such as Se, PbI2, HgI2 and the like are hardly formed into a uniform film. Therefore, the conventional planar X-ray detector of the direct conversion method has a problem that the image accuracy drops because of the film accuracy and film characteristics of the X-ray-electric charge conversion film.
As described above, the planar X-ray detector of the direct conversion method attracts attention because it provides high sensitivity. However, the X-ray-electric charge conversion film material used for the conventional planar X-ray detector is demanded that its use is decreased because it has an adverse effect on human bodies and environments and has a problem that a uniform film cannot be formed from it. Degradation of the film accuracy of the X-ray-electric charge conversion film degrades the image accuracy of the planar X-ray detector. Therefore, it is demanded that the X-ray-electric charge conversion film can be formed uniformly by using a material which does not affect adversely on human bodies and environments.
The present invention has been made in view of the above circumstances and provides an X-ray detector and an X-ray examination apparatus using it, by applying materials, which do not exert an adverse effect on human beings and environments and provide a uniform film with ease, to the X-ray-electric charge conversion film, to decrease environmental loads and the like and also to improve the X-ray detection sensitivity, detection accuracy and the like.